Issue #70: Add duration template handling

About 10k articles are using Template:Duration to specify time durations (e.g. songs or albums).
Issue #70 collects some of those.

With this change Template:Duration is parsed and a time duration in seconds is generated.
Issue #70: Add test cases for Template:Duration and Template:Height

- Added a number of test cases for Template:Duration and Template:Height based on their docs
- Fixed a problem with Template:Height parsing when either ft or in params are not defined
  (as per the template docs those cases are valid hence must be supported)
Merge pull request #105 from ninniuz/70_duration_template

Issue #70: Add duration template handling